CD52 is a glycosylated, glycosylphosphatidylinositol (GPI)-anchored cell surface protein found in abundance (500,000 molecules/cell) on a variety of normal and malignant lymphoid cells (e.g., T and B cells). See, e.g., Hale et al., J Biol Regul Homeost Agents 15:386-391 (2001); Huh et al., Blood 92: Abstract 4199 (1998); Elsner et al., Blood 88:4684-4693 (1996); Gilleece et al., Blood 82:807-812 (1993); Rodig et al., Clin Cancer Res 12:7174-7179 (2006); Ginaldi et al., Leuk Res 22:185-191 (1998). CD52 is expressed at lower levels on myeloid cells such as monocytes, macrophages, and dendritic cells, with little expression found on mature natural killer (NK) cells, neutrophils, and hematological stem cells. Id. In all, CD52 is present on at least 95% of all human peripheral blood lymphocytes and monocytes/macrophages (Hale G, et al., “The CAMPATH-1 antigen (CD52),” Tissue Antigens, 35:178-327 (1990)). CD52 is also produced by epithelial cells in the epididymis and duct deferens, and is acquired by sperm during passage through the genital tract (Hale et al., 2001, supra; Domagala et al., Med Sci Monit 7:325-331 (2001)). The exact biological function of CD52 remains unclear but some evidence suggests that it may be involved in T cell migration and co-stimulation (Rowan et al., Int Immunol 7:69-77 (1995); Masuyama et al., J Exp Med 189:979-989 (1999); Watanabe et al., Clin Immunol 120:247-259 (2006)).
Several anti-CD52 monoclonal antibodies have been developed. Campath-1H® (also known as alemtuzumab, Campath®, MabCampath®) is a humanized anti-human CD52 monoclonal antibody that exhibits potent in vitro cytotoxic effects (antibody-dependent cell mediated cytotoxicity (ADCC) and complement-dependent cytotoxicity (CDC)). Alemtuzumab recognizes an epitope that consists of the carboxy terminal four amino acids of the mature CD52 protein and a portion of the negatively charged GPI anchor. Additional anti-human CD52 monoclonal antibodies have been generated. However, the binding affinity of some of these antibodies decreases in storage and under certain pH and temperature conditions. Thus, a need exists for anti-CD52 antibodies that have a reduced propensity to undergo this change.